The present invention relates to a proportional control electromagnetic relief valve. More particularly, the present invention relates to a proportional control electromagnetic relief valve having an improved means to position and hold a valve needle in the proper orientation, so that the needle is maintained in proper contact with a seat.
Referring now to FIG. 2, an example of a conventional proportional control electromagnetic relief valve is shown. A proportional control electromagnetic solenoid 1 is attached to the upper part of a valve body 12. One end of a needle 3 is disposed at the end of a push rod 2 which is moved by solenoid 1 in the axial direction, and a pilot stage seat portion 5 of a seat 4 fitted in valve body 12 is disposed at the other end of needle 3. An orifice 14 is formed at the center of seat 4. The inner wall of an axially extending hole bored through an adapter 13 serves as a guide portion 22 for guiding needle 3.
A main poppet 6, capable of freely sliding in valve body 12 in the axial direction, is disposed below seat 4. An orifice 11 is formed at the center of main poppet 6. Main poppet 6 is biased against a main seat 7 by a return spring 8. Main seat 7 is located between an inlet port 9 and a tank port 10, which are both bored in the lower part of valve body 12 in such a manner that inlet port 9 and tank port 10 extend in the axial direction and in the radial direction, respectively. A seat inserting portion 21 for inserting and fitting seat 4 in a fluid tight state is formed on the inner face of valve body 12, at a location above a spring chamber 20 for fitting return spring 8.
Conduit passages 18a and 18b are bored through adapter 13 and valve body 12, respectively, so that the space located between seat 4 and adapter 13 communicates to the outside of valve body 12, in this case a tank T. In other words, the space between seat 4 and adapter 13 which extends in the radial direction communicates with the inside of tank T through these conduit passages 18a,18b.
Shims 15 are sandwiched between solenoid 1 and valve body 12 so that the operating distance of needle 3 with respect to that of push rod 2 can be adjusted by increasing or reducing the number of shims 15. Thus, the set pressure can be adjusted. The space between solenoid 1 and valve body 12 is kept fluid-tight by an O-ring 16. Seat 4 is fastened to the inner side of valve body 12 with screw-in type adapter 13. There is a space 19, extending in the axial direction, between seat 4 and adapter 13. The upper end of adapter 13 is held by a stop ring 17.
The operation of the conventional proportional control electromagnetic relief valve is now described. First, solenoid 1 pushes needle 3 through push rod 2 with a thrusting force F.sub.1 which is proportional to a current value I (expressed in amperes). When push rod 2 pushes needle 3, thereby pressing the end of needle 3 against pilot stage seat portion 5 of seat 4, thrusting force F.sub.1 and current value I are in the following proportional relationship, as illustrated in FIG. 3: EQU F.sub.1 =k.sub.1 .multidot.I (1)
where k.sub.1 is a proportional constant.
Now, A.sub.p represents the area receiving the pressure at the end of needle 3 fitted in seat portion 5 of seat 4, while A.sub.m represents the area of main poppet 6 which receives the pressure, both at the upstream side and downstream side of poppet 6 with respect to orifice 11. A preset load of return spring 8 is represented by F.sub.2.
Where an inlet pressure P.sub.IN at inlet port 9 is lower than F.sub.1 /A.sub.p, inlet pressure P.sub.IN and the pressure in spring chamber 20 are equal, because seat portion 5 is closed. Therefore, main poppet 6 is pushed against main seat portion 7 by return spring 8, and the communicative passage between inlet port 9 and tank port 10 is blocked.
When P.sub.IN increases to the state where P.sub.IN =F.sub.1 /A.sub.p (2), needle 3 is pushed upward, thereby opening seat portion 5 so that pilot fluid flows from inlet port 9 through orifice 11, spring chamber 20, orifice 14 of seat 4 and then through conduit passages 18a, 18b into tank T at a pilot flow rate Q.sub.1. Granting that the tank pressure is zero for the sake of convenience, the difference in pressure between the areas in the front and rear of orifice 11, i. e. the differential pressure .DELTA.P represented by the following equation (3), is generated: EQU Q.sub.1 =k.sub.2 .multidot.A.sub.3 .multidot.(.DELTA.P).sup.1/2( 3)
where k.sub.2 and A.sub.3 represent a proportional constant and a cross section of the aperture of orifice 11, respectively.
When the pressure acting on main poppet 6 with respect to the preset load F.sub.2 of return spring 8 becomes F.sub.2 .ltoreq.A.sub.m .multidot..DELTA.P, main poppet 6 is pushed upward, thereby opening the passage between inlet port 9 and tank port 10 while maintaining the state represented by equation (2). In other words, inlet pressure P.sub.IN is set at a desired value by current value I in accord with equations (1) and (2).
A conventional proportional control electromagnetic relief valve described above has needle guide portion 22 in adapter 13. Guide portion 22 limits the undesirable inclination of needle 3 away from an axial direction. Guide portion 22 therefore prevents pressure hunting and improper pressure rise which may result from improper seating of needle 3 in seat 4 due to off-center contact when the end of needle 3 is pushed by push rod 2 against seat portion 5.
The gap between guide portion 22 and needle 3 must be kept to a minimum. Otherwise, guide portion 22 would fail to function properly, permitting needle 3 to incline to a large degree. The discrepancy in axial alignment between any two coaxial members is hereinafter referred to as the degree of noncoaxiality. It is unavoidable that the degree of coaxiality between guide portion 22 of adapter 13 and seat portion 5 of seat 4 is disadvantageously large, because it is an accumulation of respective degrees of noncoaxiality of other members. Reducing the clearance between guide portion 22 and needle 3 to a small distance may cause the end of needle 3 to abut against only one side of seat portion 5, and cause pressure hunting or improper pressure rise. Consequently, reducing the clearance between guide portion 22 and needle 3 can often produce a result opposite to that intended.
The aforementioned accumulation of respective degrees of noncoaxiality of various members includes (i) a degree of noncoaxiality between the end of needle 3 and the outer perimeter of needle 3, (ii) a degree of noncoaxiality between the inner diameter of guide portion 22 of adapter 13 and the outer threaded portion of adapter 13, (iii) a degree of noncoaxiality between an adapter-mounting threaded portion 27 of valve body 12 and seat insertion portion 21 in which the outer surface of seat 4 is fitted, and (iv) a degree of noncoaxiality between seat portion 5 and the outer circumferential surface of seat 4. In particular, the coaxialities of (ii) and (iv) above are especially prone to undesirable deviation from a coaxial orientation.
For this reason, the conventional configuration requires some degree of clearance between guide portion 22 and needle 3. For example, the clearance between guide portion 22 and needle 3 in the conventional device of FIG. 2 is approximately 0.3 mm. Therefore, it is desirable to provide a means to ensure that needle 3 remains in the proper orientation with respect to seat portion 5, to minimize pressure hunting or improper pressure rise.